Description, reliability and utility of a ground-reaction-force triggered protocol for precise delivery of unilateral trip-like perturbations during gait

Tripping is a common cause of falls and a focus of many biomechanical investigations. Concerns regarding the precision of delivery of simulated-fall protocols reside in the current biomechanical methodology literature. This study aimed to develop a treadmill-based protocol that generated unanticipated trip-like perturbations during walking with high timing precision. The protocol utilized a side-by-side split-belt instrumented treadmill. Programmed treadmill belt acceleration profiles (two levels of perturbation magnitude) were triggered unilaterally at the instant the tripped leg bore 20% of the body weight. Test-retest reliability of fall responses was examined in 10 participants. Utility was examined as to whether the protocol could differentiate the fall recovery responses and likelihood of falls, estimated using peak trunk flexion angle after perturbation, between young and middle-aged adults (n = 10 per group). Results showed that the perturbations could be precisely and consistently delivered during early stance phases (10–45 milliseconds after initial contact). The protocol elicited excellent reliability of responses in both perturbation magnitudes (ICC = 0.944 and 0.911). Middle-aged adults exhibited significantly greater peak trunk flexion than young adults (p = 0.035), indicating that the current protocol can be utilized in differentiating individuals with different levels of fall risks. The main limitation of the protocol is that perturbations are delivered in stance rather swing phase. This protocol addressed some issues discussed in previous “simulated fall” protocols and may be useful for future fall research and subsequent clinical interventions.


System Apparatus Design System Apparatus Design
A Bertec side-by-side split-belt instrumented treadmill (Model ITC-11-20L-4, Bertec Corp., Columbus, OH, USA) has independent control of the movements of the two treadmill belts. Each belt is equipped with one force plate capturing GRF data from the walker's foot contacts. The force data are sampled at 1000Hz, timesynchronized with the VICON (Oxford Metrics, Oxfordshire, UK) motion data and streamed by the Software Development Kit (SDK) to MATLAB (MathWorks Inc., Natick, MA, USA) on a personal computer. The MATLAB program serves as the control interface to communicate between the VICON Datastream SDK and the treadmill controller. Concisely, the MATLAB program reads the vertical GRF (vGRF) from Datastream SDK, and when the pre-determined conditions are met it executes the pre-programmed perturbations via the treadmill controller. The treadmill controller receives remote control commands from MATLAB via the Transmission Control Protocol/Internet Protocol port, through which the program delivers the prescribed tripping perturbation by accelerating/decelerating the treadmill motors ( Figure 1 and 2B).

Perturbation Design Perturbation Design
The treadmill-based tripping perturbation protocol begins with establishing participants' comfortable walking speeds (CWS). During a perturbation a designated treadmill belt, either left or right, decelerates for 50ms, followed by 270ms of acceleration, and then decelerates again for 220ms to return to the CWS ( Figure 2B). This profile is designed to simulate the sudden blockage of a foot followed by the momentary forward thrust of the body center of mass. Two acceleration levels are used to simulate tripping perturbations of two levels of magnitude (small vs. large) (Sessoms et al., 2014) . The acceleration magnitude utilized in the protocol is linearly scaled by the CWS. For a CWS at 1 m/s, the acceleration is either ± 6 m/s 2 (small tripping perturbation) or ± 12 m/s 2 (large tripping perturbation). This allows the delivery of more realistic magnitudes of perturbation for individuals with slower walking speeds. The magnitude of perturbation may be adjusted to the researcher's needs. The automatic triggering criteria are based on the vGRF profile with the intention to deliver the perturbation precisely during early stance phase of the tripped limb. It ensures that the tripped limb went through the full course of the velocity changes of the treadmill belt. The perturbation is triggered when the following conditions are jointly met: First, the vGRF of the tripped side has to be between 20-25% of the person's body weight. Second, vGRF that met the first condition has to be greater than the vGRF 10ms prior to ensure that the trigger would occur in the ascending phase of the vGRF typical of during the early stance phase (Figure 2A).   We suggest that this protocol should only be applied to individuals who can walk independently without assistive device for at least 5 minutes. The suggestion is estimated based on the time required for familiarizing the individual to treadmill walking, fitting for the harness, and completing the protocol.

SAFETY WARNINGS
Given the protocol is triggered by a vertical ground reaction force during gait, the protocol is not applicable to individuals who are unable to walk on a treadmill. The safety of the individual should be the priority.
Potential injury, pain, or discomfort may be induced by protocol in individuals with prior lower extremity orthopedic issues due to the sudden movement of the leg. Additional warning message from the treadmill manufacturer as related to the execution of the protocol is provided below: "Once remote control for the treadmill is enabled, the treadmill can execute immediate full-speed motion based on commands received from the remote source. Please ensure you have proper firewall settings in place to prevent uncommanded treadmill operation. Manual operation of treadmill controls will overwrite and disable remote control." protocols.io | https://dx.doi.org/10.17504/protocols.io.ewov1o8k7lr2/v1

BEFORE START INSTRUCTIONS
Please double check the setting for fall prevention/participant protection. We strongly suggest a safety harness to be installed and properly tested before applying the protocol to any individual. To ensure participants do not hit the ground, after fitting participants to the harness, we asked them sit in the harness and put their full body weight to it like playing on the swing and bending their knees. We adjusted the tether under this circumstance so that their knees would not hit the ground even in the most severe fall event. The adjusted tether length should not interrupt the participant's gait. Padding can be added to the supporting struts and handrails to better protect the participants. In order to run the protocol, a few parameters need to be gathered including participants' comfortable walking speeds for treadmill walking in m/s (or a designated speed decided by the research team) and their body weight in kg.